In the field of ultra-high density magnetic recording, the magnetic field sensors used since 1992 in the read heads of computer hard disks are based on magneto-resistive materials which allow variations in magnetic field at the surface of the magnetic media to be converted into voltage variation at the terminals of the magneto-resistive sensor. So that there is good impedance matching with the pre-amplifier which pre-amplifies the sensor signal prior to the signal processing chain, research is being conducted into producing sensors which have impedance of the order of a few tens of Ohms (and typically 30 Ω). Given the decreasing size of the sensor related to the increase in the storage density, this pre-supposes working with materials which have Resistance-Area (RA) products typically of between 0.1 Ω·μm2 and a few Ω·μm2.
A plurality of magneto-resistive materials has been envisaged:    1) entirely metal multi-layers with giant magneto-resistance (see for example: J. Bass and W. Pratt, Journ. Magn. Magn. Mater. 200 (1999) 274). These offer resistance levels that are for the moment too low, typically of the order of between 1 and a few tens of mΩ·μm2;    2) To increase the RA product, a proposal has been made to introduce into the separating film a discontinuous oxide film the effect of which is to locally confine the current lines (an approach termed “current confined path”, or CCP) (Nagasaka, K et al, Journ. Appl. Phys., 89, 6943 (2001)). However, this approach raises reliability problems with the magneto-resistive sensor. Indeed, current densities through the pinholes of the discontinuous oxide film are considerable (and typically 109 to 1011 A/cm2). This leads to excessive electro-migration phenomena which may affect the lifespan of the sensors (presumed to be >10 years).    3) Huge progress has been made over recent years in respect of magneto-resistive tunnel junctions with the result that RA products can now be obtained that are sufficiently low with simultaneously a sufficient magneto-resistance amplitude. These junctions are produced either alumina based (typically RA˜1 to 5 Ω·μm2 with TMR between 10 and 30%), or MgO based (typically RA˜7 to 50 Ω·μm2, TMR˜50 to 150%), or TiOx based (typically RA˜0.3 to 2 Ω10 μm2, TMR˜10 to 20%). However, the lateral dimension of the sensor is getting ever smaller and this characteristic RA product needs to be reduced further in order to be able to preserve a sensor resistance in the right range. But the usual materials AlOx, MgO do not allow the RA product to be lowered sufficiently (tunnel barrier height too high as explained below). New materials are therefore required for the tunnel barrier with a lower barrier height.
This same need for materials with a low barrier height exists in the context of spin-transfer radio-frequency oscillators. These oscillators include magnetic tunnel junctions wherein magnetization excitations are generated that are maintained by using the so-called “spin transfer” phenomenon. These excitations typically appear at current densities (J) above a few 107A/cm2. M the tunnel junctions are voltage-limited as a result of electrical breakdown phenomena (typically to ˜0.5 volt), it will be understood that this sets a limitation on the maximum value of the RA product at a few Ω·μm2 since V=RA·J (Ohm's Law). These values are difficult to reach in a reliable and reproducible way and without significant loss of tunnel magneto-resistance amplitude with materials like AlOx or MgO. Here too, it is felt there is a need to have materials available for the tunnel barriers that have a lower barrier height, and therefore a better transparency for the tunnel electrons.
There is renewed interest being shown in MRAM (Magnetic Random Access Memories) due to the fact that magnetic tunnel junctions are being perfected that have a strong magneto-resistance at room temperature. These magnetic tunnel junctions comprise a stack of a plurality of alternately magnetic and non-magnetic films.
Preferentially, the two magnetic films located on either side of the non-magnetic film acting as a tunnel barrier, are produced based on 3d metals (Fe, Co, Ni) and alloys thereof. These films are also capable of being doped with boron or zirconium, in order to render the structure of said films amorphous and to level the interface thereof.
The tunnel barrier is an insulating or semi-conducting film, and therefore separates the two aforementioned magnetic films. It more often than not comprises amorphous alumina (AlOx) or crystalline magnesium oxide (MgO).
In the context of magnetic memories, the two magnetic films located on either side of the tunnel barrier are in the locked state and free state, respectively.
“Locked” is taken to mean that the orientation of the magnetization of said film is fixed even though the orientation of the magnetization of the so-called “free” film is capable of varying under the effect of an external magnetic field for example, the respective orientation of the magnetizations of the free and locked films defining the state of the resulting memory.
In order to lock the so-called “locked” magnetic film, there is a known technique of coupling same to an additional magnetic film produced in an antiferromagnetic material, such as for example one produced on the basis of manganese alloy, and more specifically PtMn, PdPtMn, NiMn, IrMn or FeMn.
If in terms of the physical principle, the operation of these magnetic tunnel junctions has been demonstrated, there is on the other hand a difficulty with industrialisation to be overcome. Indeed, research is being conducted now on combining for the magnetic tunnel junctions employed, whatever use they are put to (magnetic memory, oscillator or read head), a high value of the magneto-resistance, defined by the TMR magnitude (ratio of the tunnel magneto-resistance between the parallel and anti-parallel configurations of the magnetizations of the so-called free and locked films), which are therefore required to be above 200%, and a low value of the RA product, and typically below 1 Ω·μm2.
There is a plurality of reasons why this combination is significant:                in the field of mass storage on hard disk, increasing the storage density requires a reduction in the size of the read head in order to read the ever finer information written on the disk. As far as the tunnel junctions are concerned, this translates into an increase in their resistance if their RA product is not adapted. To maintain constant head impedance around 50Ω, it is therefore necessary to gradually reduce the RA product. For densities of the Tb/in2 and beyond, the necessary RA products must be <<1 Ω·μm2. In parallel, it is desirable for the read signal not to be degraded, and therefore for the TMR to remain high. But it is known to those skilled in the art that if the thickness of the tunnel barrier is reduced below 0.8 nanometres, the uniformity if not the continuity of the barrier is no longer guaranteed, which causes a rapid drop in the TMR.        in the memory field, it is the write mode that requires the RA product to be reduced to extremely low values. Indeed MRAMs such as STT-RAM (Spin Transfer Torque Magnetic Random Access Memory) are written simply by causing a high density spin polarized current to flow through the tunnel junction. When the current density exceeds a threshold value (˜107 A/cm2), the reversal of the magnetization of the free film can be observed, and without any assistance from a magnetic field. To avoid a breakdown of the tunnel barrier, it is desirable for its RA product to be significantly reduced, generally below 5 Ω·μm2. The more quickly it is required to write, the more the current density increases. It is therefore considered necessary to drop below 1 Ω·μm2. As for the read heads, the memory output signal is directly proportionate to its TMR. It is therefore appropriate to keep it at a high level while reducing the RA product.        
Prior art magnetic tunnel junctions in fact have a great many limitations.
First of all, for magnetic tunnel junctions with MgO barrier of low RA product, i.e. close to 0.5 Ω·m2, the value of the TMR is relatively small, and typically close to 50% because of the inhomogeneity effects of the barrier thickness, which reduces the read (voltage) signal and creates hot points in the tunnel barrier (zone of highest current density), increasing the risks of electrical breakdown of the tunnel junction.
Furthermore, the height of such tunnel barriers is significant, and typically close to 7.5 eV, so that very low RA products cannot in this way be attained, while guaranteeing good reliability (risk of breakdown given the very small degree of barrier thickness required).
What is more, complex oxidation methods are encountered in producing the tunnel barrier since more often than not the starting point is a pure magnesium target.
In order to overcome these difficulties, a proposal has been made to replace the tunnel barrier, more often than not made of crystalline MgO, by polycrystalline strontium titanate (SrTiO3).
Theoretically speaking, the use of such a material as tunnel barrier leads to the following advantages:                high TMR (above 300%);        low RA product<<1 Ω·μm2;        reduced barrier height (<4.5 eV);        simplified development method (starting from SrTi oxide and not metal).        
However, given the crystallization temperature of the strontium titanate, it has not hitherto been possible to develop such magnetic tunnel junctions. Indeed; this crystallization temperature remains very high, above 500° C., and therefore not compatible with the materials employed for the magnetic electrodes located on either side of the tunnel barrier.
Indeed, a crystalline SrTiO3 barrier is traditionally made by physical vapour deposition (PVD) such as cathode sputtering for example or by laser ablation and generally requires elaboration (deposition or annealing) temperatures typically of between 550 and 800° C., which is consequently incompatible with the magnetic films constituting the stack.